The present invention relates to environmental control systems (ECSs), and more particularly to an ECS ram air fan subsystem.
ECSs provide a supply of conditioned air to an enclosure, such as an aircraft cabin and cockpit. Conventional ECSs have utilized an air-to-air cycle cooling system which is in a heat exchange relationship with a liquid loop. The liquid loop typically cools other heat loads such as avionics packages. Interaction between the air and liquid subsystems is relatively complex. Moreover, airflow sequencing, particularly for multi-turbine air cycle machines (ACMs), radically affects ECS efficiency. In many instances much thermal energy may be wasted or otherwise inefficiently used.
In one conventional system, a flow of bleed air is taken from an intermediate or high pressure stage within a jet engine. The bleed air is pre-cooled within an air-to-air heat exchanger with heat being rejected to ram air and then flowed to a compressor of the ACM. After compression, the air is routed through a second air-to-air heat exchanger, a regenerative heat exchanger and an air-to-air reheater heat exchanger. Condensed water vapor is extracted by a water extractor, and dehumidified air is routed to a turbine. Expanded air from the turbine flows through another water collector and into a liquid-to-air heat exchanger of the liquid loop. A relatively warmer liquid in the liquid loop which is used to cool the avionics is thereby cooled. From the liquid-to-air heat exchanger, the air passes through the reheater heat exchanger. The dehumidified air is then passed into a second turbine of the ACM where it is again expanded and passed through another liquid-to-air heat exchanger to further absorb heat from the liquid loop.
Ram air fan subsystems operate when the ram airflow may not be sufficient to provide sufficient airflow over the air-to-air heat exchanger. Conventional air driven ram air fan subsystems utilize a fan rotor upstream of the air-to-air heat exchanger supported by conventional roller bearing elements and/or a fan rotor downstream of the air-to-air heat exchanger supported by air bearings. Operation of the ram air fan subsystem may generate some temperature rise across the air-to-air heat exchanger which reduces the efficiency thereof. This may reduce the efficiency of the entire ECS system.
Accordingly, it is desirable to provide an airflow sequence for an ECS system which more efficiently utilizes bleed air as a cooling medium and which minimizes the inefficiency of the ECS ram fan system.